According to My Hero Academia, the fundamental nature of heroism isnt just saving others, its inspiring others to be heroes as well. All Might performs strength because he knows his image is important - he cant just get the job done, he must make others believe in the power of heroism. Midoriya initially catches All Mights eye precisely because he inspires All Might to remember the spirit of heroism - and when Midoriya falters in the U.A. placement exam, its the fact that he once again inspired another to action that gets him through. The core of heroism resides in a heros ability to make others believe in themselves - to pass on their spirit like One For All, on to the next hero down the line.